The Global Position System (GPS) is a wide-spread use to provide global position for navigation and developed by satellite triangulation positioning principle. The GPS has capable of broadcasting the satellites' coordinates in the air and the clock times. A receiver unit of the GPS that is mounted on the ground can calculate the distance from a satellite to the receiver by measuring the transmission time of the satellite radio signal. By calculating aforementioned information, e.g. distance and coordinate of each satellite, the receiver unit can obtain three-dimensional (3D) space coordinate and get two possible position ranges. By synchronizing to a fourth satellite, the receiver unit can detect the time difference and decide the final one from two possible positions.
The pseudo-range differential is the most commonly used calibration method. The reference stations are set up to observe the satellites. Reference station has a known coordinate. According to the given coordinate of the reference station and the coordinate of satellite, the real distances between the reference station and satellite can be computed. The reference station also measures the distance of the pseudo-range. Thus, the correction of pseudo-range (pseudo-range difference) can be calculated.
The Differential Global Positioning System (DGPS) is proposed and planned to improve the pseudo-range difference. The principle of DGPS adopts the observation of reference station of known coordinates and no far from the receiver as a calibrator. The calibrator and the receiver receive GPS satellite signals simultaneously. The calibration is computed by the calibrator according to known latitude and longitude. Thus, the neighboring client that receives the calibration message will get meter-level accuracy. In municipal complex environment, for example urban, the multipath interference difference will not be the same at different geographical position